Remove FluentValidation; validator/DI cleanup (#7)

## Summary
- Remove FluentValidation dependency; replace with plain IModelValidator<T>/ValidationResult pattern (ported from BuyEngine's Guard/validation approach), wired via a ModelValidationActionFilter
- Move FeatureUsage code into MicCheck.Api.Features.Usage namespace
- Convert logic-free data classes to records per CLAUDE.md (WebhookEvent, FeatureStateResult, ProjectPermissionRequirement, AuditLog, Tag, FeatureUsageDaily)
- Extract IAuditService interface, drop virtual-method mock seam on AuditService
- Split Program.cs DI registrations into per-namespace DependencyRegistration classes

## Test plan
- [x] dotnet build (API + tests) clean
- [x] dotnet test MicCheck.Api.Tests.Unit — 646/646 passing
- [x] pre-push hook (full solution build/test + admin build) passed

Co-authored-by: miccheck-ci <ci@miccheck.local>
Reviewed-on: #7

tests/api/MicCheck.Api.Tests.Unit/Features/Usage/FeatureUsageControllerTests.cs

@@ -0,0 +1,93 @@
+using MicCheck.Api.Data;
+using MicCheck.Api.Features.Usage;
+using MicCheck.Api.Tests.Unit.TestSupport;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.Extensions.Caching.Memory;
+using Moq;
+using NUnit.Framework;
+
+namespace MicCheck.Api.Tests.Unit.Features.Usage;
+
+[TestFixture]
+public class FeatureUsageControllerTests
+{
+    private Mock<IMicCheckDbContext> _db = null!;
+    private List<FeatureUsageDaily> _usage = null!;
+    private IMemoryCache _cache = null!;
+    private FeatureUsageController _controller = null!;
+    private const int EnvironmentId = 1;
+
+    [SetUp]
+    public void SetUp()
+    {
+        _db = new Mock<IMicCheckDbContext>();
+        _usage = [];
+        _db.SetupDbSet(c => c.FeatureUsageDaily, _usage);
+        _cache = new MemoryCache(new MemoryCacheOptions());
+
+        _controller = new FeatureUsageController(new FeatureUsageQueryService(_db.Object), _cache);
+    }
+
+    [Test]
+    public async Task WhenGettingDashboardUsageWithNoData_ThenEmptyResultsAreReturned()
+    {
+        var result = await _controller.GetDashboardUsage(EnvironmentId);
+
+        var ok = result.Result as OkObjectResult;
+        var response = (DashboardUsageResponse)ok!.Value!;
+        Assert.That(response.TopFeaturesLastDay, Is.Empty);
+    }
+
+    [Test]
+    public async Task WhenGettingDashboardUsageASecondTime_ThenTheCachedResultIsReturnedWithoutQueryingAgain()
+    {
+        var first = await _controller.GetDashboardUsage(EnvironmentId);
+        _usage.Add(new FeatureUsageDaily
+        {
+            EnvironmentId = EnvironmentId,
+            FeatureId = 1,
+            FeatureName = "flag",
+            UsageDate = DateOnly.FromDateTime(DateTime.UtcNow),
+            Count = 100,
+            UpdatedAt = DateTimeOffset.UtcNow
+        });
+
+        var second = await _controller.GetDashboardUsage(EnvironmentId);
+
+        var firstOk = (DashboardUsageResponse)((OkObjectResult)first.Result!).Value!;
+        var secondOk = (DashboardUsageResponse)((OkObjectResult)second.Result!).Value!;
+        Assert.That(secondOk, Is.SameAs(firstOk));
+    }
+
+    [Test]
+    public async Task WhenGettingDashboardUsageForDifferentDayRanges_ThenEachRangeIsCachedSeparately()
+    {
+        var sevenDays = await _controller.GetDashboardUsage(EnvironmentId, days: 7);
+        var fourteenDays = await _controller.GetDashboardUsage(EnvironmentId, days: 14);
+
+        var sevenOk = (DashboardUsageResponse)((OkObjectResult)sevenDays.Result!).Value!;
+        var fourteenOk = (DashboardUsageResponse)((OkObjectResult)fourteenDays.Result!).Value!;
+        Assert.That(sevenOk, Is.Not.SameAs(fourteenOk));
+    }
+
+    [Test]
+    public async Task WhenDaysExceedsTheMaximum_ThenItIsClampedTo90()
+    {
+        _usage.Add(new FeatureUsageDaily
+        {
+            EnvironmentId = EnvironmentId,
+            FeatureId = 1,
+            FeatureName = "flag",
+            UsageDate = DateOnly.FromDateTime(DateTime.UtcNow),
+            Count = 5,
+            UpdatedAt = DateTimeOffset.UtcNow
+        });
+
+        var clamped = await _controller.GetDashboardUsage(EnvironmentId, days: 1000);
+        var explicitNinety = await _controller.GetDashboardUsage(EnvironmentId, days: 90);
+
+        var clampedOk = (DashboardUsageResponse)((OkObjectResult)clamped.Result!).Value!;
+        var explicitOk = (DashboardUsageResponse)((OkObjectResult)explicitNinety.Result!).Value!;
+        Assert.That(clampedOk.DailyUsage.Count, Is.EqualTo(explicitOk.DailyUsage.Count));
+    }
+}